D. VERDIN,S. M. HYDE,AND F. NEIGHBOUR
1992
parameters est,imated by the method of Pitzer were T, = 1185"K., P, = 83 atm., V , = 310 cc. (and acentric factor = 0.32). These compare with the experimental values reported above: T, = 1220°K. and V , = 302 CC. The agreement of the estimated T , and V , with the experimental values is good. This indicates that the correspondence between critical constants (T, and V,) and liquid density and vapor pressures observed for many molecular fluids by Riedel also obtains for bismuth bromide (as for bismuth chloride). Pitzer? indicates that a more stringent test of a normal fluid can be made with the surface tension. Using the measured T , and V,, 'we calculated from Pitzer's eq.? A 1-20 a value for uo of 148 dynes/cm. and from his eq. A 1-19 and the measured values of T, and
density a value of 75 dynes/cm. for the surface tension of the liquid a t 250". An experimental value of 66.5 dynes/cm. was given by Jaeger.g Pitzer' indicates that a deviation of the calculated surface tension of more than 5% from the estimated value indicates significant deviation from normal behavior. Therefore, although bismuth bromide behaves like a molecular fluid with regard to its critical volume and temperature, it may be expected to deviate from "normal" behavior in some of its other properties.
Acknowledgment. This work was supported jointly by the Research Division of the U. S. Atomic Energy Commission under Contract No. AT(04-3)-106 and Stanford Research Institute. (9) F. M. Jaeger, 2. anorg. allgem. Chenz., 101, 176 (1914).
The Radiation-Induced Oxidation of p-Xylene Sensitized by Organic Bromine Compounds
by D. Verdin, S. M. Hyde, and F. Neighbour Wantage Research Laboratory, United Kingdom Atomic Energy Authority, Wantoge, Berkshire, England (Received December 89,1964)
The oxidation of p-xylene a t 25" induced by 6oCo7-radiation is accelerated by the addition of organic bromine compounds. CBr4 is the most efficient sensitizer, and the rate of absorption of oxygen accelerates to a maximum a t about 2% oxidation and then decreases. The observed kinetics are consistent with a mechanism in which the autoretardation and the postirradiation oxidation result from thereaction, ROOH HBr -+ RO. HzO Br., between the hydroperoxide and hydrogen bromide formed as intermediates.
+
Introduction The oxidation of p-xylene to terephthalic acid is normally carried out in organic acid solution in the presence of metal bromides a t temperatures above 120O.1 The metal ion catalyzes the conversion of the transient peroxy EXh&3 to an aldehyde, and the bromine atoms formed abstract a hydrogen atom from The Jozkrnal of PhySieal Chemistry
+
+
the methyl group in the intermediate p-toluic acid much more readily than peroxy radicals are able to do.2 In an attempt to increase the rate of oxidation of P-xylene h-rmm." and Ohtaa investigated the influence (1) G. H. Whitfield, British Patent 837,321 (1960). (2) D. A. s. Ravens, Trans. Faraday SOC., 55, 1768 (1959).
RADIATION-INDUCED OXIDATION O F
1)-XYLENE
of 6oCo y-radiation on the reaction. At 80' no acceleration of the reaction occurred on irradiating, but at 135' irradiation doubled the rate of hydroperoxide formation and increased the production of acids. However, no terephthalic acid was obtained, and the conversion did not exceed 5 mole % owing to the production of an inhibitor. Irradiation of the much faster cobalt naphthenate catalyzed oxidations of p-xylene or p-toluic acid caused negligible changes. Costea, et al.,4 oxidized p-xylene a t temperatures from 60 to 130' using reactor irradiation, but even a t the higher temperature they found that conversion to peroxide and p-toluic acid was less than 2% after 40 hr. However, with 1% cobalt naphthenate catalyst, t,hese authors found that irradiation gave a yield of 20% p-toluic acid in 8 hr. under conditions where negligible reaction occurred in the absence of radiation. Bakh5 found that the radiation-induced oxidation of toluene a t 25' gave a low yield of peroxides, the total 0 value being 1.8. Peroxy radicals do not readily abstract primary hydrogen atoms from alkyl benzenes6 so that a siniilar nonchain mechanism is expected for the radiation-induced oxidation of pure p-xylene a t 25". However, bromine atoms have a low activation energy for such abstractions7 and may be able to induce a chain oxidation at room temperature, similar to that observed in gas phase oxidations catalyzed by HBr, in which the following propagating sequence occurs.*
+ RH +HBr + R. R * + +ROz. R02. + HBr -+ROOH + BrB r a
0 2
We have therefore examined the 6oCoy-radiationinduced oxidation of p-xylene containing organic bromine compounds at 25'. The radiolysis of the additive should produce a steady supply of bromine atoms which could possibly maintain the oxidation sequence to high conversions of the p-xylene.
Experimental iMaterials. ?,-Xylene (Phillips Petroleum Co., pure grade) was repeatedly shaken with concentrated H2S04 until the acid layer was colorless. It was then washed with 10% Na2C03solution and several times with distilled water, then dried with anhydrous CaS04. After repeated shaking with mercury until tarnishing was insignificant, it was fractionally distilled in a nitrogen atmosphere through a 72-theoretical-plate column. The distillate was passed through a 12-cm. colunm of chromatographic alumina, collected in a
1993
blackened flask, and stored under nitrogen in a greasefree automatic buret. The boiling point range 138.36 f 0.03' (corrected to 760 mm.) agreed with the literature value,g and gas-liquid chromatographic analysis revealed no detectable impurities. The rate of the radiation-induced oxidation of the above p-xylene was unchanged after three recrystallizations. Oxygen from a cylinder was passed through columns of silica gel and KOH and then through a trap a t -78'. Carbon tetrabromide (Hopkin and Williams Ltd.) was recrystallized twice from ethanol and twice from petroleum ether (b.p. 40-60') and then sublimed according to the procedure of Bradley and Drury.'O The absorption spectrum of the purified material was unchanged on further sublimation and had a maximum at 229 mp with a molar extinction coefficient of 4.66 X lo3in Spectrosol hexane a t 20.0'. Bromobenzene, bromoform, ethyl bromide (treated to remove alcohol and waterg), carbon tetrachloride, and bromotrichloromethane were fractionally distilled in a nitrogen atmosphere in a 35-plate all-glass column shielded from light. Small middle fractions were collected a t boiling points agreeing with published value^.^ Immediately after distillation the required amounts of the liquids were degassed in small ampoules with break-seals and stored in the dark until used. Hydrogen bromide (Matheson Co., Inc., anhydrous) was degassed by freezing and thawing cycles and then stored in a blackened bulb until used. A calibrated bulb was filled a t 20.0' to pressures measured with a mercury manometer, and the known amounts of HBr were condensed into the reaction vessel. Similar condensation of the HBr into excess alkali, followed by titration, confirmed its purity as 99.9%. Benzyl alcohol and cobalt naphthenate (Hopkin and Williams Ltd.) and DzO (L. Light and Co. Ltd., 99.5%) were used as supplied. Measurement of Oxygen Absorption. The rates of oxygen absorption a t selected pressures were measured (3) J. Imamura and N. Ohta, Tokyo Kogyo Shikensho Hokoku, 55, 346 (1960). (4) T. Costea, C. Mantesco, and I. Negoesco, Intern. J . AppZ. Radiation Isotopes, 13, 306 (1962). (5) N. A. Bakh, Symposium on Radiation Chemistry, Academy of Sciences of the U.S.S.R., English Translation, Consultants Bureau, NewYork, N. Y., 1956, p. 119. (6) G. A. Russell, J . A m . Chem. Soc., 78, 1047 (1956). (7) H. R. Anderson, H. A. Scheraga, and E. R. Van Artsdalen, J . C h a . Phys., 2 1 , 1258 (1953). (8) F. F. Rust and W. E. Vaughan, Ind. Eng. Chem., 41, 2595 (1949). (9) A. Weissberger, et aZ., "Technique of Organic Chemistry," Vol. V I I , Interscience Publishers, Inc., New York, N. Y., 1955. (10) R. S. Bradley and T. Drury, Trans. Faraday Soc., 55, 1844 (1959).
Volume 60, Number 6 June 1066
D. VERDIN,S. M. HYDE,AND F. NEIGHBOUR
1994
automatically in a constant pressure apparatus.l' Oxidations were performed in cylindrical glass reaction vessels of 30-mm. internal diameter and 10-mm. internal thickness, and having a 7-mm.-diameter side tube incorporating a diaphragm break-seal and a narrow bore side arm. Measured amounts of previously degassed p-xylene and additives were distilled or sublimed under vacuum into the reaction vessel through the side arm, which was then sealed off. The reaction vessel was immersed in a thermostat controlled to *0.02O and clamped to a bar a t reproducible positions with respect to a 230-curie 6oCosource12which was also immersed in the thermostat. To keep the solutions saturated with oxygen the reaction vessel was continuously shaken a t measured rates by means of a motor driving a Kopp Variator (Allspeeds Ltd.) connected, by an eccentrically mounted rod, to the bar supporting the reaction vessel. The latter was connected to the oxygen absorption apparatus via a flexible glass spiral. After evacuating the system and filling with oxygen to the required pressure, the break-seal was fractured by a glass-covered magnet, and the apparatus controls were set before introducing the 6oCo source to start the reaction. The rate of oxygen absorption was independent of the shaking speed from 188 to 655 c./min.; measurements were normally made at a rate of about 540 c./min. Dosimetry. The rates of energy absorption in the p-xylene solutions were calculated on the basis of electron densities from measurements made in the reaction vessels with the ferrous sulfate dosimeter, taking Gw+ = 1.5.5,13and correcting for 6oCodecay. Analysis. The formation of reaction products was investigated in the reaction vessels used for the oxygen absorption runs, but having a stoppered side arm added for sample withdrawal, the oxygen pressure being maintained at about 850 nim. Hydroperoxides were measured by the iodometric method of Wibaut, et u Z . , ~ ~and it was shown that CBr4 and benzyl alcohol present in the amounts used in this work did not affect the results of analyses using pure t-butyl hydroperoxide. Samples were analyzed immediately after removal from the radiation source. Infrared spectra of the reaction products were measured after removing p-xylene and volatile products by vacuum distillation.
This rate corresponds to an observed G value for oxygen absorption of 2.70. However, the value must be corrected for the radiolytic formation of Hz and CHI which occurs in the presence of 0 2 and amounts to GH, G C H = ~ 0.2216 under the irradiation conditions used here, so that the true rate of oxygen absorption is G(-02) = 2,92. To determine the relative influence of a range of halogen-containing additives, oxidations were carried out at 25" and approximately the same dose rate. The additives were all present at the same electron fraction of the solutions, so that the proportion of the radiation energy absorbed in them should be the same, and the oxidation rates should be determined by their radical yields and the efficiency of these radicals in establishing oxidation chains. Plots of 0 2 absorbed as a function of time (radiation dose) for some of the additives are compared with that for p-xylene in Figure 1. The corresponding plot for CHBrs resembled those of CBr4 and CC13Brin showing an initial period of accelerating rate, while those for CzHsBr, CGHSBr,and CC1, all showed an initial oxidation rate which started to decrease in the region 0.5-l% oxidation. The maximum rates of oxygen absorption are summarized in Table I.
Results
(11) F.Neighbour and D. Verdin, J . Sci. Instr., 41,219 (1964). (12) G. S. Murray, R. Roberts, and D. Dove, Radioisotopes in Scientific Research, Proceedings of the 1st International Conference, Paris, Sept. 1957,Vol. 1, Pergamon Press Ltd., London, 1958,p. 139. (13) R. H.Schuler and A. 0. Allen, J . Chem. Phys., 24, 56 (1956). (14) J. P.Wibaut, H. B. van Leeuwen, and B. van der Wal, Rec. trav. chim., 73, 1033 (1954). (15) D. Verdin, J . Phys. Chem., 67, 1263 (1963).
Irradiation of p-xylene at a dose rate of 1.93 X lo1* e.v. 1.-l sec.-l in the presence of oxygen at 25" resulted in oxygen being absorbed at a rate of 8.65 X mole I.-' sec.-l (mean of four runs) which was constant to an absorbed dose of at least 1.04 X loz4e.v. 1.-1. The Journal of Physical Chemistry
+
Table I: Maximum Oxidation Rates of pXylene Solutions of Halogen Compounds a t 25.0" (electron fraction of additives = 0.0420)
a
Additive
Dose rete, e.v. 1.-1 8ec.-1 x 10-18
None CnH5Br C6HSBr CHBr3 CCI, CC13Br CBr4
1.93 1.88 2.00 1.91 1.99 1.92 1.96
-d(Oz),
moles 1. -1 dt sec.-I X lo' Maximum G+02)a
0.865 4.59 8.81 9.27 3.41 12.3 20.3
2.92 14.9 26.8 29.5 10.5 38.6 62.6
Corrected for Hz and CH, production.
Sensitization by CBr4. It is seen that CBr4 is the most effective additive for increasing the oxidation rate, and the kinetics of this system were therefore studied in more detail, the rate of oxidation always
RADIATION-IKDUCED OXIDATIONOF XYLENE
1995
/
d 0.4
t
e
x
b
3.5
t
CC1,Br
%
9 0.3
4
h
n
c
2 -:
0.1
Y
I
/ / ~
20
40
60
80
100
4 8 12 16 Dose rate, e.v. I.-' set.-* X 1017.
120
Time, hr.
Figure 1. Absorption of O2by p-xylene solutions at 25.0'; dose rat,e = 1.92 f 0.04 x 1018 e.v. 1.-1 see.-', electron fraction of additives = 0.0420.
for the maximum rate of oxygen absorption, where I is the dose rate. The variation of reaction rate with temperature was determined from measurements of the rate of oxygen absorption a t three dose rates a t each temperature. The linearity of reaction rate with dose rate persisted to the highest temperature used, and the intercept at zero dose rate increased with temperature (Figure 2). The runs were all performed at a CBr4 concentration of 0.141 M , and the slopes of the plots of -d(02)/ dt against I were divided by (CBr4)0.26 to obtain kobsd. An Arrhenius plot of the values of kobsd summarized
24
Figure 2. Variation of maximum rate of oxidation of 0.141 M CBrasolutions in p-xylene with dose rate and temperature.
3.5
being measured as the maximum slope of the oxygen absorption curve. The maximum rate of oxidation at 25" was independent of the oxygen pressure above the solution over the range 411-1077 mm., the rates being measured at a dose rate of 1.05 X lo1*e.v. I.-' sec.-l and a CBr4 concentration of 0.141 M . Oxidation rates were normally measured at pressures of about 850 mm. The rate of oxygen absorption increased linearly with the radiation dose rate (Figure 2) over the range 8.41 X 1OI6 to 2.08 X lo1*e.v. 1.-l sec.-l at 25.0", and extrapolation of this plot to zero dose rate gave a finite rate of oxygen absorption (Ro). The manner in which the reaction rate depended on the concentration of CBr4 is shown in Figure 3. The highest concentration employed corresponds to an electron fraction of CBr4 in the solution of 0.154. A logarithmic plot of the data leads to the expression
~~~
20
1
I
0.1
1
I
0.2
0.3
1
0.4
f
I
0.5
0.6
(CBr4), M .
Figure 3. Effect of CBr4 concentration on oxidation rate a t 25.0'; dose rate = 2.08 X lo1*e.v. sec.-l.
in Table I1 leads to an over-all activation energy of 8.1 kcal./mole for the maximum rate of oxidation. The formation of hydroperoxides during the oxidation is compared with the absorption of oxygen under identical conditions in Figure 4. Infrared spectra of the nonvolatile reaction products, which were yellow colored, after 22- and 70-hr. irradiation closely resembled Table lI: Temperature Dependence of Rate Constant for Maximum Rate of Oxygen Absorption Temp., O C .
kobsd X 1024, moleQ441.0.26 e.v.-l
25.0 40.0 55.0 70.0
1.63 3.23 4.94 9.90
Volume 69, Number 6 June 1966
D. VERDIN, S. n1. HYDE,AND F. NEIGHBOUR
1996
20
60
40
80
100
Time, hr.
Figure 4. Comparison of 02 absorption and hydroperoxide formation in the oxidation of 0.141 M CBrl in p-xylene at; 25.0"; dose rate = 1.74 X 10'8 e.v. 1.-1 set.-', point A = 1.57% oxidation.
that of benzyl alcohol, including a strong band at 3500 cm.-l, which confirms the appearance of OOH and/or OH groups16in one of the methyl groups of p-xylene. The ultraviolet absorption spectrum of the oxidate after 25-hr. irradiation showed a peak at 229 mp due to CBr4, but no quantitative measure of the CBr4 concentration was possible owing to the high absorption of p-xylene in this region. Postirradiation Oxidation. Removal of the radiation source during the course of an oxidation resulted in a marked decrease in the rate of oxygen absorption. Subsequent reintroduction of the source did not immediately restore the reaction rate to its value prior t o removal of the source, but there was a gradual acceleration in rate. The postirradiation rate depended on the degree of oxidation at which the irradiation was stopped in the manner shown in Figure 5, and at each conversion the rate decreased with time and eventually became zero. The postirradiation rate attained its highest values in the same conversion range as the maximum rate of oxidation occurred, that is, between 1.0 and 2.5% oxidation, and at conversions exceeding 3.6% oxidation there was no posteffect. The reaction product has been found to contain hydroperoxide, and t o determine whether this could give rise to the postirradiation oxidation, a 2.6 x A l solution of pure &butyl hydroperoxide in p-xylene was shaken with 0 2 ; however, the solution absorbed no 0 2 in 41 hr. at 25". HBr is a probable intermediate in the reaction and also might cause the postirradiation effect. To examine this, reaction vessels were employed in which p-xylene was saturated with 0 2 before breaking the seal of a compartment into which The Journal of Physieal Chemistry
0.5
1.5 2.0 2.5 3.0 Oxidation of pxylene, %.
1.0
3.5
4.0
4.5
Figure 5. Postirradiation oxidation of 0.141 M CBr4 in p-xylene after radiation-induced oxidation to various conversions at 2 5 O ; dose rate = 1.89 X 1018 e.v. I.-' sec.-I.
HBr had been distilled. These solutions, which were 0.087 M with respect to HBr, absorbed 0 2 very rapidly mole at 25" (at initial rates of approximately 5 X 1.-1 sec.-l), after an induction period of about 3.5 hr. The reaction rate decreased rapidly and was zero after 1.8% oxidation. The induction period was reduced to less than 5 min. by irradiating the solutions at a dose rate of 6.8 X lo1' e.v. 1.-l sec.-l, and after the reaction had started removal and reintroduction of the radiation source had no effect on the oxidation rate. Moreover, if the reaction was allowed to go to completion in the absence of radiation, there was no further absorption of O2 on irradiating or introducing more IIBr. If HBr was added in the above manner (to give a concentration of 0.073 M ) to a 2.6 X low3M solution of &butyl hydroperoxide in p-xylene, a similar rapid absorption of 0 2 occurred with no induction period and at initial rates of approximately 1.6 X loe4 mole 1.-1 sec.-l. The reaction rate decreased as the reaction proceeded and became zero after 1.0% oxidation. If solutions of these concentrations of HBr and hydroperoxide were mixed in the reaction vessel and left for 24 hr. before admitting 0 2 , there was an induction period of 14 hr. before 0 2 was absorbed at an mole 1.-l sec.-l, which fell initial rate of 2.0 X to zero after 0.59% oxidation. The initial rate of the reaction and the final extent of oxidation both increase as the initial concentrations of HBr and hydroperoxide are increased, e.g., with HBr = 0.26 M and hydroperM the initial rate of O2absorption oxide = 5.4 X exceeded 3.8 X mole 1.-l set.-', and the reaction (16) G. J. Minkoff, Proc. Roy. SOC.(London), A224, 176 (1954).
RADIATION-INDUCED OXIDATION O F p-XYLENE
had almost stopped absorbing 0% after 10 hr. when 3.9% oxidation had occurred. The initial rates of 0 2 absorption in these solutions were insufficiently reproducible to permit kinetic study of the system, probably owing to the reaction rate being comparable to the rate of diffusion of 0 2 into the liquid phase, and distribution of HBr between the two phases may also contribute in this way. It was observed that the addition of t-butyl hydroperoxide (to give a concentration of 0.06 M ) to a 0.141 M solution of CBr4 in p-xylene resulted in the absorption of oxygen. However, the rate was negligible compared with the postirradiation oxidation which occurs a t conversions corresponding to this hydroperoxide content in the radiation-induced oxidation of p-xylene containing the same concentration of CBr4. The above experiments demonstrate that a reaction capable of initiating oxidation occurs between HBr and t-butyl hydroperoxide or the hydroperoxide produced when p-xylene containing 0 2 and HBr is irradiated. Moreover, the reaction forms a product which acts as an oxidation inhibitor. The production of inhibitors in the CBr4 sensitized oxidation was confirmed by oxidizing p-xylene containing 0.141 M CBr4until it reached 3.9% conversion and then adding a fresh portion of CBr4 and continuing the oxidation. The maximum rate of 0%absorption in this second stage was less than 30% of the maximum rate of the original oxidation. Retardation of Oxidation. One of the probable reaction products is p-methyl benzyl alcohol and it is desirable to know to what extent it can retard the oxidation. Benzyl alcohol should exhibit a very similar degree of retardation and it was used for this test in view of its commercial availability. The oxidation rate as :t function of the concentration of benzyl alcohol added st the beginning of the runs is shown in Figure 6. The slight increase in oxidation rate after the rapid initial fall is probably due to direct oxidation of the benzyl alcohol at the higher concentrations absorbing an appreciable amount of oxygen. Water also exhibits a retarding effect on the oxidation and this is illustrated in Figure 6. The saturation solubility of water in xylene at 25” is 0.02 M,” so that after a relatively small amount of a second phase has been formed (approximately 10 times the solubility, i.e., 0.02 ml. of water in samples containing 5 ml. of pxylene), the addition of more water causes negligible further decrease in oxidation rate. To provide information on the mechanism by which water retards the oxidation its influence was compared with that of similar amounts of DzO, and these results are included on the same graph. It is seen that within experi-
1997
16.0
R
0.1 0.2 0.3 0.4 Moles of additive per 1. of pxylene.
0.5
Figure 6. Effect of benzyl alcohol (0),water (A), and D20 ( 0 ) on the maximum rate of oxidation of 0.141 M CBrp in p-xylene at 25.0’; dose rate = 1.71 X e.v. 1.-l sec.-l.
mental error the behavior of water and D2O is identical. It is significant to note that the addition of excess M solution of t-butyl hydroperwater to a 2.6 X oxide in p-xylene before the addition of HBr (to give a 0.073 M solution) entirely prevented the absorption of O2by this system, which would otherwise have undergone a rapid oxidation. The addition of cobalt naphthenate (1.77% by weight) to an oxidation which was performod under identical conditions with that illustrated in Figure 4 gave a maximum rate of oxygen absorption of 9.81 X lo-’ mole 1.-l sec.-l, which is approximately half of that in the absence of cobalt naphthenate. This rate was attained after a period of 85 hr. during which the oxidation slowly accelerated, and at the end of the run the vessel contained a precipitate of a bromine compound of cobalt. Cobalt naphthenate therefore does not function as a catalyst under the conditions used in this system.
Discussion The low G value of 2.92 for oxygen absorption by p xylene in the absence of additives and the G values for radical production from alkylbenzenes18indicate that a nonchain oxidation is occurring, as would be expected at 25” from the low reactivity of the methyl hydrogen atoms toward peroxy radicals.6 Our result is consistent with that of Bakh,5who found a total yield of peroxides of G = 1.8 in the radiation-induced oxidation of (17) A. Seidell, “Solubilities of Organic Compounds,” Vol. 11, D. Van Nostrand Go., Inc., New York, N. Y., 1941, p. 607. (18) E. N. Weber, P. F. Forsyth, and R. H. Schuler, Radiation Res., 3, 68 (1955).
Volume 69, Number 6 June 1966
D. VERDIN, S. nl. HYDE,AND F. NEIGHBOUR
1998
toluene at a dose rate of 4.8 X lOls e.v. 1.-l set.-' at 25". The G values for the absorption of 0 2 in the presence of various sensitizers (Table I) indicate that the oxidation occurs by a chain mechanism, but that the chain lengths are short. There are insufficient published data on radical yields to enable predictions to be made on the relative oxidation rates induced by these compounds, but the G(-o,) values for CzH& and CsHsBr indicate the effectiveness of Br atoms for inducing oxidation. It is improbable that the radical yield from CCLBr will exceed that from CC1, by a factor of 4, so that the fourfold higher oxidation rate (Table I) of p-xylene in the presence of CC13Br must result from the oxidation chains established by the Br atom. However the fact that CC1, does accelerate the oxidation of p-xylene implies that the Ccl3- fragment from CC13Br contributes to the G(-o,, value observed with this sensitizer. Consequently the difference in behavior between CC4Br and CBr4 would require the latter to have about twice the Br atom yield of CCLBr if CBr8 radicals did not initiate oxidation chains. I n view of the equality of C-Br bond dissociation energies in these two molecules, and the fact that dissociation energies for C-C1 bonds in such molecules are much greater than for C-Br bonds,lg it is unlikely that the Br atom yields differ by a factor of 2, and it is suggested that when CBr4 is used as a sensitizer both the Bra and CBr3. species initiate oxidation chains. The initiation steps in the oxidation may be represented by the equations
catalytic effect of HBr in the gas phase oxidation of hydrocarbons.8,21 The lifetimes of the transient radicals in the above reactions will be short, so that the postirradiation oxidation must arise from reactions between intermediate products. We have found that the reaction of HBr with a hydroperoxide can initiate the oxidation of pxylene, and we therefore include reaction 7 in the scheme to provide a source of bromine atoms to initiate the postirradiation oxidation. It has been proposed that
+ Br. CBr3. + RH +CHBr3 + R . Br- + RH +HBr + R.
with reactions 5 and 6 for peroxy radicals and so retard the oxidation. We therefore include reaction 10, and it may be noted that benzyl alcohol is an effective inhibitor of the HBr-catalyzed oxidation of benzyl bromidez2in the liquid phase a t temperatures exceeding 194".
CBr4
CBr3.
(1) (2)
(3)
Reaction 3 occurs in the side-chain bromination of toluene in the gas phase and has an activation energy of 7.2 keal./mole.7 The oxidation rate is independent of the oxygen pressure so that all the p-xylyl (R.) radicals must, react rapidly with oxygen.
R.
+
--+ROz.
(4) The activation energy for attack of peroxy radicals on p-xylene should exceed the value of 10.5 kcal./mole for p-cymene'o so that the peroxy radicals will not compete with reaction 3 but will take part in the following propagating steps. 0 2
R02. + CHBr3 +ROOH
+ CBr3.
+ HBr --+
+ Br.
ROz.
ROOH
(5)
(6)
Reactions 3, 4, and 6 are those invoked to explain the The Journal of Physical Chemistry
ROOH
+ HBr --+
RO.
+ H 2 0 + Bra
(7)
this reaction causes chain branching in the HBrcatalyzed oxidation of isobutene in the gas phase.21 The linear dependence of oxidation rate on radiation intensity indicates that the termination reactions do not involve bimolecular interaction of chain-carrying species. The retardation of the reaction at low conversions, and the complete cessation at equally low conversions of oxidations initiated by mixtures of HBr and hydroperoxide, implies that reaction 7 gives rise to this autoretardation. The infrared spectrum of the product is consistent with the presence of a benzyltype alcohol, and we found that benzyl alcohol added to the reaction mixture strongly retards the oxidation. The RO radicals produced in reaction 7 must therefore form p-methyl benzyl alcohol (ROH), probably by reactions 8 and 9, and the resulting alcohol will compete
+ CHBr3+ROH + CBrx RO. + HBr +ROH + Br.
RO.
(8)
(9)
+
R02. ROH --+ chain termination (10) Addition of water at the beginning of the oxidation markedly reduces the rate, but it is a less effective retarder than benzyl alcohol. In view of the high O-H bond dissociation energy in HzO it seems improbable that H-atom abstraction is involved in the retardation, and it is proposed that it functions by converting HBr into the ionized form, in which it will no longer take (19) N. N. Semenov, "Some Problems of Chemical Kinetics and Reactivity," Vol. I, Pergamon Press Ltd., London, 1958, p. 19. (20) H. Boardman, J. Am. Chem. Soc., 84, 1376 (1962). (21) P. Hurst, G. Skirrow, and C. F. H. Tipper, Proc. Roy. Sac. (London), A268,405 (1962). (22) R. Barnett, E. R. Bell, F. H. Dickey, F. F. Rust, and W. E. Vaughan, I n d . Eng. Chem., 41, 2612 (1949).
RADIATION-INDUCED OXIDATION OF p-XYLENE
part in reaction 6 or 7. When the solubility of HzO in p-xylene is exceeded the distribution coefficient for HBr will greatly favor its transfer to the aqueous phase when only a small amount is present. HBr
+ HzO +OHs+Br-
(11) The identical behavior of DzO and water implies that the retardation does not involve bond breakage in the water molecule and supports the proposal that ionization of the HBr causes the retardation, since no primary isotope effect in the water molecule is involved and water and DzO should have similar efficiencies in bringing about reaction 11. When only a small amount of the aqueous phase has appeared the addition of more water causes no further decrease in oxidation rate, and we also found that excess water completely inhibits oxidations initiated by the reaction between HBr and a hydroperoxide. However, in the irradiated CBr4 system, excess water reduces the oxidation rate to only half of that in the absence of added water. This is consistent with the proposed mechanism since water will not affect the oxidation chain (reactions 2, 4, and 5) initiated by CBr3 radicals, and the result implies that the sequence accounts for approximately half of the oxidation occurring. Benzyl alcohol is able to reduce the oxidation rate below that caused by the presence of excess water since it acts by scavenging peroxy radicals and thus retards the oxidation chains initiated by both Br atoms and CBr3 radicals. Although reaction 11 removes bromine from the oxidation sequence, some bromine must be removed by another process which will give rise to the product causing the yellow color of the oxidate. The low volatility of the colored product shows that it is not molecular bromine, and it may be derived from the CBr3 radical; however, the identity of the colored product was not established. If the termination reactions are first order with respect to the chain-carrying species, then the reaction rate will be proportional to the initiation rate and the observed linear dependence on dose rate is consistent with reaction 1 being the main process for initiation. It might be expected that the oxidation rate would be proportional t,o the CBr4 concentration; however, the observed dependence (Figure 3) probably reflects the occurrence of energy transfer from p-xylene to the CBr4, Or the reaction Of CBr4 with by dissociative capture to produce CBr3 and Br- ions which are subsequently neutralized. Analogous behavior was reportedz3 for some organic halogen ComDounds
1999
complex due to the interaction of the Br atoms with a-electrons of the aromatic ring24 and this could enhance the probability of energy transfer in this system. On irradiating cc14 solutions in benzene, Oster and KallmannZ5found that the transfer of energy from benzene to CCl, leads to the production of radicals at a rate which depends on the concentration of CC14 in a similar manner to Figure 3 in the same range of concentrations of the halogen compound. If the G value for loss of CBr4 is taken as 10 and it is assumed that all the energy absorbed in the solution leads to decomposition of CBr4, then, for the oxidation represented in Figure 4,36.9% of the initial CBr4 would be decomposed after 50-hr. irradiation, so that loss of CBr4 by radiolysis does not contribute significantly to the falloff in oxidation rate. By assuming stationary concentrations of all the radical species involved, the instantaneous oxidation rate may be expressed in terms of the concentrations of the reaction intermediates and products by the equation
where N is Avogadro's number and the index QI allows for energy transfer to CBr4. The reaction sequence predicts that the concentration of HBr in the p-xylene will pass through a maximum, and in the region where this occurs the oxidation rate should also be a maximum. The above rate equation indicates that this maximum rate will vary linearly with the dose rate and that the graph will extrapolate to a finite intercept at zero dose rate, and this intercept should increase with temperature because of the activation energies of the rate constants involved. Figure 2 shows that the system does behave in this way. As appreciable amounts of a separate aqueous phase form, removal of HBr progressively reduces the terms involving (HBr) and the rate of oxidation falls. Further reduction in the rate of oxygen absorption occurs as the concentration of the alcohol increases ; however, when its concentration becomes sufficiently high, the direct radiation-induced oxidation of the alcoholz6 (23) D. W. Brown and L. A. Wall, J . Polymer Sci., 44, 325 (1960). (24) F. J. Strieter and D. H. Templeton, J . Chem. Phys., 37, 161 (1962). (25) G. K. Oster and H. D. Kallmann, A'uture, 194, 1033 (1962).
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June 1965
2000
will compensate for the decrease in rate of oxygen ab-
sorption. When sufEcient water has been produced to eliminate the chain involving HBr, the CBr3 oxidation chain should continue to increase the hydroperoxide concentration. The observation that the hydroperoxide concentration falls to a lower level may be due to energy transfer from p-xylene to the hydroperoxide resulting in its decomposition. Such transfer can lead to high G values for peroxide decomposition when both the peroxide and the solvent are aromatic.27 The postirradiation oxidation is consistent with the above equation, which indicates that its rate will follow the HBr and hydroperoxide concentrations and will be zero when there is sufIicient water to remove all the HBr. The over-all activation energy of the oxidation in terms of the activation energies of the individual steps in the oxidation scheme would involve too many unknown values to pennit any comparison between the calculated and experimental values. The reaction may involve additional steps such as further reactions of the CBr3 radicals or hydroperoxide mole-
?'he Journal of Physical Chemistry
D. VERDIN,S. M. HYDE,AND F. NEIGHBOUR
cule; however, the above mechanism explains the essential features of the system. Hendry and Russell2*have recently observed that the liquid phase oxidation of cumene containing traces of HBr proceeds extremely rapidly initially but soon inhibits itself, which they assumed to be due to the formation of phenol by an acid-catalyzed decomposition of cumene hydroperoxide. However, in cumene solution the HBr would be in the covalent form, and on the basis of the present work it would be expected to react with the hydroperoxide to produce dimethylphenylcarbinol and water which would inhibit the oxidation. HBr is also known22to catalyze the liquid phase oxidation of benzyl bromide and mesitylene at temperatures in the region of 200", where the water produced would be removed in the 0 2 gas stream and so permit extensive oxidation. (27) V. A. Krongauz and Kh. S. Bagdasaryan, Zh. Fiz. Khim., 32, 717 (1958). (28) D. G. Hendry and G. A. Russell, J. Am. Chem. SOC.,86, 2371 (1984).
